JEE Main 2022 · 30 June · morning · Single correct · +4 / −1
A → ^(573 K) Red phosphorus → _(under pressure)^(heat ; 803 K) B
Red phosphorus is obtained by heating "A" at 573 K, and can be converted to "B" by heating at 803 K under pressure.
A and B, respectively, are
- (a) β-black phosphorus and white phosphorus.
- (b) white phosphorus and β-black phosphorus.
- (c) α-black phosphorus and white phosphorus.
- (d) white phosphorus and α-black phosphorus.
Answer and solution
Answer (d) white phosphorus and α-black phosphorus.
Solution
Chemistry ma’am · 3 steps
Remember the starting allotrope: alpha-black comes from red phosphorus, beta-black comes from white.
- Find A. Red phosphorus is made by heating white phosphorus at 573 K in an inert atmosphere for several days. So A is white phosphorus.
- Two black forms. Black phosphorus has two forms, and they are made from different starting materials. Which one you get depends on whether you begin with red or white phosphorus.
Start from Conditions Product White phosphorus 573 K, inert atmosphere Red phosphorus Red phosphorus 803 K, sealed tube (under pressure) α-black phosphorus White phosphorus 473 K, high pressure β-black phosphorus - So. Heating red phosphorus at 803 K under pressure gives α-black phosphorus. A is white phosphorus and B is α-black phosphorus, which is option (d).
The idea to keep. Link each allotrope to its starting material and temperature: white at 573 K gives red, red at 803 K gives alpha-black, white at 473 K under high pressure gives beta-black.
Answer from the NTA final answer key. Solution by PrepWiser.